Fully-depleted son

ABSTRACT

A semiconductor device and a method of fabricating a semiconductor device. The semiconductor device includes a semiconductor substrate, an insulating layer, a first semiconductor layer, a dielectric layer, a second semiconductor layer, a source and drain junction, a gate, and a spacer. The method includes the steps of forming a semiconductor substrate, forming a shallow trench isolation layer, growing a first epitaxial layer, growing a second epitaxial layer, forming a gate, forming a spacer, performing a reactive ion etching, removing a portion of the first epitaxial layer, filling the void with a dielectric, etching back a portion of the dielectric, growing a silicon layer, implanting a source and drain junction, and forming an extension.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.13/048,977, filed Mar. 16, 2011, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates to a semiconductor device and a method offabricating the same. More particularly, the present invention relatesto a fully-depleted silicon-on-nothing (SON) device.

Electronic devices are widely used in almost all aspects of life. Thesedevices are getting smaller; thus the circuitry inside the devices isalso being downscaled. In order to maintain this trend, new ways toreduce the size of field effect transistors, such as metal-oxidesemiconductor field effect transistors and complementary metal-oxidesemiconductors, need to be developed. The smaller circuitry also needsto perform better than its larger counterparts.

One of the developments in downscaled semiconductor technology isextremely-thin silicon-on-insulator (ETSOI) devices. ETSOI devices havea number of advantages. However, the thickness variation in ETSOI leadsto Vt roll-off and sub-threshold slope variability, especially for agate length less than 25 nm. Silicon-on-nothing (SON) devices offercontrolled, epitaxially-grown silicon channel disposed on an insulatorlayer. However, unlike ETSOI, these devices have a source and a drain ona bulk substrate, causing significant leakage current below theinsulator layer. Moreover, conventional SON devices have dopantsegregation in the floating dielectric resulting in lower effective doseand higher extension region resistance.

The proposed invention describes a fully-depleted SON device with bettershort-channel characteristics than partially-depleted/conventional SONdevices. The structure includes an undoped-SON on an undoped-thin-SOIsubstrate. The structure eliminates the need for well implants, providesbetter Vt roll-off and lower off-state current compared to conventionalSON.

SUMMARY

The present invention provides a fully-depleted silicon-on-nothingdevice and the method of fabricating such device. The present inventionetches back the floating dielectric region before the extension implantto reduce/eliminate dopant segregation in the extension region. Removalof the dielectric beneath the extension region thereby minimizes anynegative dopant segregation effects.

According to an aspect of the present invention, a semiconductor deviceis provided. The semiconductor device includes a semiconductorsubstrate, an insulating layer on the substrate, a first semiconductorlayer on the insulating layer, a dielectric layer on a portion of thefirst semiconductor layer, a second semiconductor layer around thedielectric layer, a source and drain junction within the secondsemiconductor layer, a gate on a portion of the second semiconductorlayer, and a spacer disposed around the gate and covering a portion ofthe second semiconductor layer.

According to another aspect of the present invention, a method offabricating a semiconductor device is provided. The method includes thesteps of forming a semiconductor substrate, forming a shallow trenchisolation layer around the semiconductor device, growing a firstepitaxial layer on the substrate, growing a second epitaxial layer onthe first epitaxial layer, forming a gate on the second epitaxial layer,forming a spacer disposed on each side of the gate and on the secondepitaxial layer, recessing the semiconductor substrate, first epitaxiallayer and second epitaxial layer, removing at least a portion of thefirst epitaxial layer to create a void, filling the void with adielectric material, etching back at least a portion of the dielectricmaterial, growing a semiconductor layer around the dielectric material,forming a source and drain junction on the silicon on insulator, andforming an extension surrounded by the spacer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-section of the completed fully depleted SONdevice.

FIG. 2 illustrates a cross-section of an initial semiconductorsubstrate.

FIG. 3 illustrates a cross-section of a first and a second epitaxiallayer on the initial substrate.

FIG. 4 illustrates a cross-section of a gate surrounded by spacers onthe substrate in FIG. 2.

FIG. 5 illustrates a shallow trench isolation layer around asemiconductor device.

FIG. 6 illustrates a recessing of the substrate.

FIG. 7 illustrates a void between the initial substrate and the secondepitaxial layer.

FIG. 8 illustrates a dielectric material between the initial substrateand the second epitaxial layer.

FIG. 9 illustrates the structure of FIG. 8 with a portion of thedielectric material etched back.

FIG. 10 illustrates the structure of FIG. 9 with an implant area forjunction butting.

FIG. 11 illustrates the structure of FIG. 9 with the etched areas filledwith a new material.

FIG. 12 illustrates the structure of FIG. 11 with an extension and asource and drain junction.

FIG. 13 is a flow chart illustrating the steps of fabricating the devicein FIG. 1.

DETAILED DESCRIPTION

The following describes embodiments of the present invention withreference to the drawings. The embodiments are illustrations of theinvention, which can be embodied in various forms. The present inventionis not limited to the embodiment described below, rather representativefor teaching one skilled in the art how to make and use it. Some aspectsof the drawings repeat from one drawing to the next. The aspects retaintheir same numbering from their first appearance throughout each of thepreceding drawings.

In one embodiment of the present invention, a fully-depletedsilicon-on-nothing (SON) semiconductor device is described. The fullydepleted SON device includes a silicon channel disposed on a firstinsulating layer, a gate disposed on top of the channel, and a sourceand a drain disposed on a second insulating layer. The first insulatinglayer and the second insulating layer are separated by anothersemiconductor layer. The fully-depleted SON device is configured in sucha way that when the device is off, both the channel layer and thesemiconductor layer between the first insulating layer and the secondinsulating layer are depleted to minimize the leakage current betweensource and drain.

A silicon-on-nothing device (SON) is referred to as such because theremoval of an epitaxial layer creates a void or an air tunnel isolatinga semiconductor layer from another epitaxial layer. The device does notcollapse, despite the void that is created, because it connects theactive area of the semiconductor device and is supported at both ends bythe shallow trench isolation layer. The void is filled with a dielectricmaterial to prevent it from filling with the material on the surroundingsides while the device formation is completed.

FIG. 1 depicts the fully depleted SON device. The thickness of thedifferent layers of the SON device, as indicated below, assist inachieving better device electrostatics. The aspects of the SON will bedescribed with respect to the remaining drawings.

FIG. 2 depicts the initial semiconductor substrate for fabricating afully-depleted silicon-on-nothing (SON) device. The substrate includes asemiconductor substrate (130), an insulating layer (120) and a firstsemiconductor layer (110).

The insulating layer (120) can include a single layer of insulatingmaterial or multiple layers with different insulating materials.Examples of insulating materials include, but are not limited to,silicon oxide, silicon nitride, silicon oxynitride, high-k materials, orany combination of these materials. Examples of high-k materialsinclude, but are not limited to, metal oxides such as hafnium oxide,hafnium silicon oxide, hafnium silicon oxynitride, lanthanum oxide,lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide,zirconium silicon oxynitride, tantalum oxide, titanium oxide, bariumstrontium titanium oxide, barium titanium oxide, strontium titaniumoxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, andlead zinc niobate. Preferably, the insulating layer (120) is siliconoxide (buried oxide).

The semiconductor substrate (130) can be made of such materials asgermanium, silicon germanium, silicon carbide, a compound of Group III/Velements or a compound of Group II/VI elements. Preferably, thesemiconductor substrate (130) is silicon (Si) and the firstsemiconductor layer (110) is a silicon-on-insulator (SOI) layer. Thefirst semiconductor layer (110) is preferably of a size in the range of10-30 nm.

FIG. 3 continues to build the fully-depleted SON by including a firstepitaxial layer (220) and a second epitaxial layer (210). The firstepitaxial layer (220) is preferably made of silicon-germanium. Thislayer acts as a place holder for the “nothing” between the secondepitaxial layer (210) and the first semiconductor layer (110). Thesecond epitaxial layer (210) is preferably made of Si. The firstepitaxial layer (220) and the second epitaxial layer (210) can be formedby an epitaxial growth process such as chemical vapor deposition (CVD).Preferably, the first epitaxial layer (220) has a thickness ranging from10 nm to 50 nm and the second epitaxial layer (210) has a thicknessranging from 2 nm to 10 nm. However, a thickness of either epitaxiallayer that is greater or less than the preferred thickness is alsoconceived.

Furthermore, FIG. 4 shows a gate (310), which has a spacer (320) aroundit. The gate (310) and the spacer (320) are positioned on the secondepitaxial layer (210). The gate (310) can include at least one gatedielectric layer, for example a high-k dielectric, and at least one gateconducting layer, for example a metal or conductive metallic compound.An active semiconductor device area (420) of the SON device issurrounded by a shallow trench isolation (STI) layer (410), as shown inFIG. 5. The gate (310) is shown as being above the active area.

The shallow trench isolation (STI) layer helps to prevent currentleakage in the SON device as the device is fabricated. Moreover, the STIlayer helps to hold the layers of the device in place during thefabrication of the semiconductor device. The STI layer includes etchedtrenches around the semiconductor device, which are filled with adielectric material.

In FIG. 6, the SON device is shown with the first semiconductor layer(110), the first epitaxial layer (220) and the second epitaxial layer(210) etched back. The first epitaxial layer (220) is further etchedback selective to the second epitaxial layer (210) and the firstsemiconductor layer (110) in FIG. 7. In one embodiment, the firstepitaxial layer (220) includes silicon germanium, and the secondepitaxial layer (210) and the first semiconductor layer (110) includesilicon. The first epitaxial layer (220) is etched in a gas environmentcontaining hydrochloride. At least a portion of the first epitaxiallayer (220) is fully removed between the second epitaxial layer (210)and the first semiconductor layer (110). Preferably, the entire firstepitaxial layer (220) is etched back to the second epitaxial layer(210).

A void is created between the second epitaxial layer (210) and the firstsemiconductor layer (110) after the first epitaxial layer (220) has beenetched back. The void is filled by a dielectric material (710), as shownin FIG. 8. FIG. 9 shows that at least a portion of the dielectric (710)is also etched back. The etching of the dielectric (710) reduces it to asize in the range of 10-25 nm. The dielectric (710) is etched to about3-5 nm shorter than the length of the gate (310). The thickness of thedielectric (710) assists in achieving better device electrostatics. Thedielectric (710) can include a single or multiple layered dielectricmaterials including, but not limited to, silicon oxide, silicon nitride,silicon oxynitride, and high-k dielectric. An implant (910) for junctionbutting also can be included in the SON device, shown in FIG. 10.

In FIG. 11, the remainder of the void created by the etch processes isfilled with a second semiconductor layer (1010), for example, by anepitaxial growth process. The second semiconductor layer (1010) caninclude silicon, silicon germanium, germanium, carbon doped silicon,Group II/V compound semiconductor, Group II/VI compound semiconductor,or any suitable combination of those materials. The second semiconductorlayer (1010) has a thickness in the range of 2-10 nm. To complete thefully-depleted SON device a source (1120) and a drain (1110) created inthe second semiconductor layer (1010), as shown in FIG. 12. The source(1120) and drain (1110) can be formed by implantation. Thefully-depleted SON device can be made and used with or without theimplant (910) for junction butting.

In another embodiment of the present invention, a method of fabricatingthe fully-depleted SON semiconductor device is described. The methodassists in creating a well controlled dielectric under the channel ofthe SON device. The method also creates a uniform thickness in thechannel, thus the thickness is well controlled. Therefore, there themethod prevents dopant segregation thereby allowing higher activation inthe device.

In order to fabricate the device, a semiconductor substrate should beprovided. Preferably, the substrate is a SOI substrate. In step 1210, ashallow trench isolation layer is formed around the semiconductorsubstrate. A first and a second epitaxial layer are grown in step 1215.The first epitaxial layer is grown on the semiconductor substrate andthe second epitaxial layer is grown on the first epitaxial layer. A gateis formed in step 1220 and a spacer is formed around the gate in step1225.

In step 1230, a recessing is performed in order to minimize thedimensions of the first epitaxial layer (220), the second epitaxiallayer (210) and the first semiconductor layer (110). The recessing ispreferably performed by a reactive ion etching process. The firstepitaxial layer (220) is further reduced in step 1235 by removing atleast a portion of it to create a void between the second epitaxiallayer (210) and the first semiconductor layer (110). The void is filledwith a dielectric material (710) in step 1240. At least a portion of thedielectric (710) is etched back in step 1245 in order to achieve betterperformance of the device throughout its supporting layers. A secondsemiconductor layer (1010) is grown, in step 1250, in the void createdby the etching processes. A source (1120) and a drain (1110) are formedin step 1255 and an extension is formed on the device in step 1260.

While the invention has been described in its preferred embodiments, itis to be understood that the invention is not limited to theembodiments. Rather, various changes in form and details can be madewithout departing from the spirit and scope of the invention as setforth in the claims.

The invention claimed is:
 1. A semiconductor device, comprising: asemiconductor substrate; an insulating layer on the semiconductorsubstrate; a first semiconductor layer disposed on the insulating layer;a dielectric layer disposed on a portion of the first semiconductorlayer; a second semiconductor layer disposed around the dielectriclayer; a source and drain junction disposed within the secondsemiconductor layer; a gate disposed on a portion of the secondsemiconductor layer; and a spacer disposed around the gate and coveringa portion of the second semiconductor layer.
 2. The semiconductor deviceof claim 1, wherein the first semiconductor layer has a thickness fromabout 10 nanometers to about 30 nanometers.
 3. The semiconductor deviceof claim 1, wherein the second semiconductor layer is made of a materialselected from the group consisting of: silicon, germanium, a compound ofa Group III/V elements, a compound of a Group II/VI elements, and acombination thereof.
 4. The semiconductor device of claim 1, wherein thesecond semiconductor layer has a thickness of about 2 nanometers toabout 10 nanometers.
 5. The semiconductor device of claim 1, wherein thedielectric layer is a single-layer dielectric.
 6. The semiconductordevice of claim 1, wherein the dielectric layer is a multi-layerdielectric.
 7. The semiconductor device of claim 1, wherein thedielectric layers has a thickness of about 10 nanometers to about 25nanometers.
 8. The semiconductor device of claim 1, wherein there is animplant for junction butting disposed beneath the source and drainjunction.
 9. The semiconductor device of claim 1, wherein the dielectriclayer is about 3 nanometers to about 5 nanometers shorter than thelength of the gate.
 10. The semiconductor device of claim 1, wherein thefirst semiconductor layer is a silicon-on-insulator layer.
 11. Thesemiconductor device of claim 1, wherein the dielectric layer is made ofa material selected from the group consisting of silicon oxide, siliconnitride, silicon oxynitride and a high-k dielectric.
 12. Thesemiconductor device of claim 1, wherein the gate comprises at least onegate dielectric layer and at least one gate conducting layer.